I'm going to guess we are going to call this a defective PSU.
When you attach the power cord, the green light on the back of the PSU flashes on and off at about 1 second intervals.
The PC it is attached to will not power on via the front panel button.
According the my PSU tester, it has lost the -5v line on the main motherboard connector.
The +5 drive power line is fine.
I know this is very dangerous to the computer, but with the PSU connected to AC Power, I plugged the main motherboard connector into the motherboard and the PC came alive. It ran fine until I powered it off again, at which time it would no longer power on via the front panel.
I have replaced the PSU in said machine and the operation is now normal.
I would think there is no way, nor would it really be feasible to attempt to repair the PSU given the cost, (this is a 250 watt PSU).
Thanks in advance.